In the field of mobile hydraulics several different types of systems have evolved over the years which can be classified as either open center or closed center systems. An open center system has a fixed displacement pump and circulates the entire pump flow through the control valves when not in use. A closed center system is supplied by a variable displacement pump that delivers only the flow required to operate a motor at a desired speed, while in neutral or when not in use the pump discharge is at a zero flow level and a low standby pressure. Closed center systems, also referred to as load-sensing, are more efficient than the traditional open center system, however, they are much higher in initial cost and complexity due to the use of variable displacement pumps, more complex valves and added sensing lines thoughout the system.
An open center type of system includes an inexpensive fixed displacement pump which supplies a constant flow rate to one or more directional control valves in a bank which in turn control individual motors. The pressure maintained in open center systems is that which is necessary to overcome the pressure losses in the system and operate the motors. Since the pump in a conventional open center system always operates at its maximum flow level, there is a waste of energy in its neutral or non-use position since that maximum pump discharge must flow across all of the open center passages in each of the valves in the system before returning to reservoir. In this neutral standby condition, where none of the motors are being operated, the entire output flow from the pump passes through each of the open center passages of each valve to reservoir and the power utilized in moving this high volume of oil is completely dissipated in heat. In systems with four or five functions, this energy loss becomes substantial since this high volume of oil must pass across each valve in the system.
One hybrid system for minimizing such an energy loss is the use of closed center directional control valves which have an unloading valve in the inlet section or first section. When neutrally positioned, the unloading valve dumps the entire pump flow to reservoir without havng to circulate the pump flow through each valve in the system, as for example shown in U.S. Pat. No. 3,411,295. A similar hybrid system is illustrated in U.S. Pat. No. 3,815,477 which utilizes a similar unloading valve with a series of closed center valves in a modern load sensing system.